During the "Red King Revelation" level
(Thor, Captain America, Doctor Strange and Captain Marvel in the story), you will meet the Hulk (the variant for this achievement) and have to free him from a mind control disc that is strapped to him.
Once that is done, you gain access to him when a giant metallic worm is firing at you. Waves of enemies will continue to spawn from the left and right gates of the arena endlessly at this point, so instead of continuing with the objective, use the Hulk to kill enemies until the achievement pops. It should only take a few minutes.
You should also note that that throughout this, the worm will constantly fire at you, and you will probably die a few times. The Hulk transforms back into Bruce Banner when this happens so you will need to hold down
to turn him back into the Hulk. A bit of nuisance, but if you keep an eye on the areas the worm is attacking, you should be ok.